1. Field of the Invention
This invention relates to expansion joints for industrial ducts and piping.
2. Discussion of Prior Art
Existing flexible gas seals and expansion joints for industrial ducts with gas stream temperatures over 500.degree. F. employ a standard design and construction as described in U.S. Pat. No. 4,063,755 to Merz, Dec. 20, 1977. This standard design incorporates an inner metal flow liner plate, an insulation cavity filled with high temperature insulation, and an outer gas seal fixed to flanges. The inner metal flow liner forms the inner part of the insulation cavity, and helps to protect the insulation cavity and outer expansion joint gas seal from gas stream abrasion. The inner metal flow liner plate is welded or otherwise affixed to one section of the duct. The other end of the flow liner plate is not affixed to the other mating duct section, and there is a gap of varying size that permits the mating duct sections to slide over each other as the duct moves in response to temperature induced expansion. This gap between the flow liner and the mating duct section is determined mainly by the amount of lateral or side to side movement that the duct undergoes in operation. The larger the lateral movement of the two duct sections relative to each other, the larger this gap between the flow liner and mating duct section. The longitudinal movement of the duct does not have an effect on this gap. This standard design works well with gas streams that are not dust or particulate laden.
In dusty or particulate laden gas streams, this gap between the flow liner and the mating duct section permits the entry of dust and particulates into the insulation cavity between the flow liner and the outer gas seal expansion joint. Even though the cavity is usually filled with insulation material, the dust and particulates can build up, in some cases quite rapidly. As the mating duct sections expand and contract towards each other over time, the dust/particulates build up in the insulation cavity and become hard packed to such an extent that the outer flexible gas seal or expansion joint ruptures prematurely from the pressure of the built up dust/particulates. This problem is particularly acute in horizontal ducts with very fine dust or particulates. Complicated systems of tightly overlapping metal flow liner plates as described in U.S. Pat. No. 3,730,566 to Kasmierski et al., May 1, 1973 have been tried but are both expensive and difficult to apply, will not work in cases where there are both large torsional and large lateral or side to side duct movements, and still incorporate small gaps between the sliding metal liners that permit dust build up in the insulation cavity. Packings placed in the gap between liner and mating duct have also been tried but would have a tendency to become dislodged, would not have enough elasticity to form a tight seal after repeated duct movements, and would not work at all in cases of large lateral or side to side duct movements. Another possible solution is the incorporation of cleanout holes into the cavity for periodic cleaning of the dust build up, but this becomes a maintenance problem and will not generally work because of the hard packing of the dust/particulates.